1. Field of the Invention
The present invention relates to an evaporative fuel treatment apparatus for an internal combustion engine, and more particularly to an evaporative fuel treatment apparatus for an internal combustion engine that supplies evaporative fuel as the fuel for startup.
2. Background Art
A technology disclosed, for instance, by Japanese Patent JP-A No. 280532/1999 (hereinafter referred to as “Patent Document 1”) emits fuel vapor (evaporative fuel) adsorbed by a canister, supplies the fuel vapor to a surge tank, and supplies the fuel vapor and fresh air to a combustion chamber at internal combustion engine startup. Since the wall surface temperature is still not high during or immediately after startup, the fuel injected from a fuel injection valve is not likely to evaporate so that stable combustion is not readily accomplished. The fuel vapor exhibits excellent ignitability because it is completely gasified. Therefore, when the fuel vapor is used as the fuel for startup, the startability of an internal combustion engine improves.
Another technology disclosed, for instance, by Japanese Patent JP-A No. 36937/1999 (hereinafter referred to as “Patent Document 2”) connects a fuel tank to an intake pipe with an evaporative fuel path and directly supplies fuel vapor in the fuel tank to the intake pipe. This technology aims at maintaining a negative pressure within the fuel tank. This purpose is achieved by regulating the opening of a control valve installed in the evaporative fuel path in accordance with the pressure within the fuel tank with a view toward transferring the fuel vapor from the fuel tank to the intake pipe.
However, the technology disclosed by Patent Document 1 cannot easily adjust the amount of fuel injection from the fuel injection valve. The amount of fuel vapor emission from the canister depends on the amount of fuel vapor adsorption by the canister. In some cases, therefore, only very low concentration fuel vapor may be supplied. In some other cases, very high concentration fuel vapor may be supplied. Under these circumstances, it is necessary to estimate the amount of evaporative fuel adsorption by the canister and control the amount of fuel injection from the fuel injection valve in accordance with the estimation result. The technology disclosed by Patent Document 1 estimates the amount of evaporative fuel adsorption in accordance with the learning value of an air-fuel ratio feedback correction coefficient. At internal combustion engine startup, it estimates the amount of evaporative fuel adsorption in accordance with a learning value determined by feedback control exercised for the last operation. While the internal combustion engine is stopped, however, the fuel vapor may be adsorbed or emitted by the canister. Therefore, the amount of evaporative fuel adsorption estimated from the last learning value may significantly differ from the actual amount of evaporative fuel adsorption. Even if the amount of fuel injection by the fuel injection value is determined according to the amount of evaporative fuel adsorption estimated from the last learning value in the above circumstances, an improper combustion state may occur because a desired amount of fuel cannot be supplied. The use of a sensor makes it possible to measure the fuel vapor concentration, but entails an additional cost.
If, on the other hand, the fuel vapor in the fuel tank is to be supplied to the intake pipe as described in Patent Document 2, the concentration of the fuel vapor can be more or less determined at all times in comparison to the concentration of fuel vapor emission from the canister. However, the technology disclosed by Patent Document 2 emits the fuel vapor into the intake pipe in order to maintain a negative pressure within the fuel tank at all times. Therefore, if a negative pressure already is produced within the fuel tank or prematurely produced, it is impossible to supply a sufficient amount of fuel vapor.